[CallSite] Make construction from Value* (or Instruction*) explicit.

CallSite roughly behaves as a common base CallInst and InvokeInst. Bring
the behavior closer to that model by making upcasts explicit. Downcasts
remain implicit and work as before.

Following dyn_cast as a mental model checking whether a Value *V isa
CallSite now looks like this: 
  if (auto CS = CallSite(V)) // think dyn_cast
instead of:
  if (CallSite CS = V)

This is an extra token but I think it is slightly clearer. Making the
ctor explicit has the advantage of not accidentally creating nullptr
CallSites, e.g. when you pass a Value * to a function taking a CallSite
argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234601 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/ArgumentPromotion.cpp

@@ -862,7 +862,7 @@ CallGraphNode *ArgPromotion::DoPromotion(Function *F,
 
     // Update the callgraph to know that the callsite has been transformed.
     CallGraphNode *CalleeNode = CG[Call->getParent()->getParent()];
-    CalleeNode->replaceCallEdge(Call, New, NF_CGN);
+    CalleeNode->replaceCallEdge(CS, CallSite(New), NF_CGN);
 
     if (!Call->use_empty()) {
       Call->replaceAllUsesWith(New);

lib/Transforms/IPO/DeadArgumentElimination.cpp

@@ -482,7 +482,7 @@ DAE::Liveness DAE::SurveyUse(const Use *U,
       return Result;
     }
 
-    if (ImmutableCallSite CS = V) {
+    if (auto CS = ImmutableCallSite(V)) {
       const Function *F = CS.getCalledFunction();
       if (F) {
         // Used in a direct call.